More European genetics! This was published in Nature. Don't worry about the top map. The middle one shows genetic diversity in the milk-producing genes of European cattle. The bottom one shows the geographic distribution of lactose tolerance in Europe. Darker shades of orange indicate greater diversity.
So why is this so interesting? Because the high level of genetic diversity in north-central European cattle is an indication that that's where cattle have been around the longest: that's the area where their genes have had the most time to diversify, like an old tree whose branches have had a lot of time to spread out. And see that dashed black line in the bottom map? That indicates the area where the fossil record shows that people have been herding cattle the longest. So the genetic record and the archaeological record really strongly agree: cattle have been around for a long time in north-central Europe and southern Scandinavia.
Now look at the bottom map - the one that shows lactose tolerance in the human population. Of course, all humans digest milk as babies. But it's only thanks to some relatively new genes that many of us are able to digest milk as adults. These are the genes that make us lactose tolerant. And where are those genes concentrated? In almost the exact same area as the oldest European cattle! In other words, Europeans have evolved to be able to drink milk in just those areas where that adaptation has been most advantageous.
Those original cattle herders - the Funnel Beaker culture - established themselves aroundd 5,000 to 6,000 years ago. So, as with skin color, we're not talking about a real long time in which these evolutionary changes have occurred. Nonetheless, the population today in The Netherlands and Sweden is more than 99% lactose tolerant.
Europe isn't even the only place lactose tolerance evolved. It's happened even more recently in Africa, among three distinct populations in Kenya and Tanzania, as well as Sudan, and as recently as 2,700 years ago. (No word on lactose tolerance in India, but given that they eat a lot of dairy and have had domesticated cows the longest, there's probably some interesting genetic history there as well.)